The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the reduction of patient motion during an MRI scan and to the reduction of image artifacts caused by such motion.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The present invention will be described in detail with reference to a variant of the well known Fourier transform (FT) imaging technique, which is frequently referred to as "spin-warp". The spin-warp technique is discussed in an article entitled "Spin-Warp NMR Imaging and Applications to Human Whole-Body Imaging" by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (G.sub.y) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (G.sub.x) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse G.sub.y is incremented (.DELTA.G.sub.y) in the sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
In a three-dimensional implementation of the spin-warp method phase encoding of the spin-echo signals is performed along two orthogonal axes. As described in U.S. Pat. No. 4,431,968 entitled "Method of Three-Dimensional NMR Imaging Using Selective Excitation," a thick slab of spins is excited by applying a slab-selection gradient (G.sub.z) in the presence of a selective RF excitation pulse and then a first phase encoding gradient (G.sub.z) along the same axis and a second phase encoding gradient (G.sub.y) are applied before the NMR signal acquisition in the presence of a readout gradient (G.sub.x). For each value of the G.sub.z phase encoding gradient, the G.sub.y hase encoding is stepped through all its values to sample a three-dimensional region of k-space. By selectively exciting a slab, NMR signals are acquired from a controlled 3-dimensional volume.
MR angiography (MRA) has been an active area of research. The time-of-flight (TOF) method of MRA uses the motion of the blood relative to the surrounding tissue as a contrast mechanism. The most common approach is to exploit the differences in signal saturation that exist between flowing blood and stationary tissue. This is known as flow-related enhancement, but this effect is misnamed because the improvement in blood-tissue contrast is actually due to the stationary tissues experiencing many excitation pulses and becoming saturated. Flowing blood, which is moving through the excited section, is continually refreshed by spins experiencing fewer excitation pulses and is, therefore, less saturated. The result is the desired image contrast between the high-signal blood and the low-signal stationary tissues.
When 3D imaging methods are employed to produce an MRA image, the size of the excited slab becomes a limiting factor. To improve the diagnostic utility of the MRA image it is desirable to increase the slab thickness to increase the field of view along the slab-select axis. However, time-of-flight (TOF) MRA images decrease in quality as the slab thickness increases due to the saturation of the spins as they flow through the excited slab. That is, due to the increased thickness of the excited slab, blood remains in the slab for a longer time and becomes saturated by the selective RF excitation pulse. As a result, fresh blood entering the slab appears much brighter in the reconstructed image than blood which has remained in the slab for a number of excitations.
One solution to this problem is to acquire NMR data from the desired three-dimensional region by sequentially exciting a series of thin slabs and concatenating the NMR data acquired therefrom. As described in U.S. Pat. No. 5,167,232 entitled "Magnetic Resonance Angiography By Sequential Multiple Thin Slab Three Dimensional Acquisition," the thin slabs are often overlapped because slices on each slab boundary suffer from signal loss due to imperfect slab excitation profiles. Without this thin slab overlap, a "Venetian blind" artifact is normally produced.
Overlapping thin slabs are acquired by sampling some of the locations along the thin slab boundaries twice. The final image of the overlap region is obtained by using a maximum intensity projection (MIP) of each reconstructed image data set.
A problem with acquiring overlapping image data sets is that they cannot be properly registered if the patient moves during the time period between the thin slab acquisitions. Such mis-registration can produce vessels which appear "double" in the overlap region, and this is a severe artifact that can result in a misdiagnosis.